This invention relates to a molecular sensor for sensing molecules by Raman spectroscopy and a Raman spectroscopic process.
It is known that Raman light can be amplified by the plasmons generated on the surface of a metal thin film.
For example, J. Appl. Phys., Vol. 88, No. 11, pp. 6187-6191, Dec. 1, 2000, describes Raman spectroscopy of thin films enhanced by surface plasmon polaritons. In this article, utilizing the surface plasmon resonance of a silver thin film formed on a glass substrate, Raman spectroscopy is carried out on a TiO2 thin film on the Ag thin film.
In FIG. 4 of the article, Configurations A, B and C are illustrated as the optical setup for Raman spectroscopy. In Configuration A, a prism is located beneath the glass substrate, laser light is incident on the Ag thin film from the prism for total reflection at the lower surface of the Ag thin film, and Raman light is detected at the top of the TiO2 thin film. Configuration B is identical with Configuration A in that the prism is located beneath the glass substrate, but laser light is incident on the Ag thin film from above the TiO2 thin film. Configuration C is identical with Configuration B in that laser light is incident on the Ag thin film from above the TiO2 thin film, but the prism is removed. Of the three optical setups, Configuration A affords the highest intensity of Raman light. No Raman spectrum is detected from Configuration C without the prism.
It is understood from the above article that a total reflection prism must be used in order to amplify Raman light by surface plasmon resonance. However, the provision of a total reflection prism requires to increase the size of device. Also, the necessity to set a fixed incident angle of laser light decreases the freedom of design of the device configuration.
On the other hand, JP-A 2000-356587 describes a localized plasmon resonance sensor. This sensor includes a substrate and metal fine particulates secured to the substrate surface in film form and having a diameter of 10 to 20 nm. The advantage alleged therein is that utilizing the localized plasmons generated in proximity to surfaces of metal fine particulates, the sensor dispenses with a total reflection prism. The metal fine particulates secured to the substrate surface in film form are formed by immersing a glass substrate in a 10% methanol solution of 3-aminopropyltrimethoxysilane for 10 minutes, washing the substrate and immersing it in a gold colloidal solution for 2 hours.
The sensor of the above-referred JP-A 2000-356587 is an affinity sensor which measures the absorbance of light transmitted by metal fine particulates for thereby detecting the refractive index of a medium in proximity to the metal fine particulates, and thereby determines whether any substance is adsorbed to the metal fine particulates. It is not a Raman spectroscopic sensor capable of determining the species of molecules.
A study of the present inventors has revealed that a practical level of sensitivity is not reached when Raman spectroscopy is carried out using the sensor of the above-referred JP-A 2000-356587. This is presumably because in the gold fine particulate monolayer film formed by binding gold colloid to the glass substrate surface, the distance between gold fine particulates arrayed is too large, which results in generation of less localized plasmons and hence, insufficient amplification of Raman light. In addition, the gold fine particulate monolayer film formed by binding gold colloid, the distance between gold fine particulates arrayed is relatively large and the uniformity of array density is relatively low as seen from FIG. 5 (SEM photomicrograph) of the patent publication. This also accounts for insufficient amplification of Raman light.
An object of the present invention is to reduce the size and increase the sensitivity of a molecular sensor which carries out Raman spectroscopy utilizing the amplification of Raman light by plasmons, and to provide a high sensitivity Raman spectroscopic process.
In a first aspect, the invention provides a molecular sensor comprising a sensor film containing a metal compound wherein Raman spectroscopic analysis is carried out utilizing the amplification of Raman light by the localized plasmons that fine particles resulting from reduction of said metal compound generate.
In a preferred embodiment, the sensor film has been formed by physical vapor deposition. Preferably, the metal compound is reduced by irradiating electromagnetic wave or with exciting light used in the Raman spectroscopic analysis. The metal compound is preferably a metal oxide or metal halide and typically contains Ag.
In a second aspect, the invention provides a Raman spectroscopic process utilizing the amplification of Raman light by the localized plasmons that fine particles consisting essentially of a metal generate. Preferably the fine particles are formed by reduction of a metal compound.